my 25c worth

what i've heard, and my experience with mod proofs seems to support this, is that the mint's casing can cause the toning (which, is a form of corrosion, insomuch as it is a chemical reaction between the silver, or whatever else, and the outside world, ultimately changing the coin). now, when i've sent my coins with mint-bequeathed toning, they have all been graded (one even ended up with a PCGS 70). thus, at least without seeing them, this is what i can tell you.